This invention relates to a liquid crystal display, and more particularly to a liquid crystal display and a method for operating the liquid crystal display, which can improve the picture quality by reducing voltage variation of pixel electrodes due to both a parasitic capacitor between a gate electrode and a source electrode of a thin film transistor that operates each pixel and a leakage current of the transistor.
A conventional active matrix type liquid crystal display includes an underside plate having transistors for switching pixels and pixel electrodes arranged thereon, an upperside plate having color filters for filtering color, a common electrode, liquid crystal filled between the underside and upperside plates, and two polarizing plates attached on outer sides of the underside and upperside plates for linear polarization of visible lights.
The conventional liquid crystal display will be explained hereinafter, with reference to the attached drawings.
FIG. 1 is an equivalent circuit of the conventional liquid crystal display. FIG. 2 shows a voltage characteristic curve of a pixel electrode of the conventional liquid crystal display shown in FIG. 1. FIG. 3 is an equivalent circuit of a unit pixel region of the conventional liquid crystal display shown in FIG. 1.
The conventional liquid crystal display includes a plurality of gate lines N.sub.1 to N.sub.n arranged in one direction each spaced apart from one another by a fixed interval, a plurality of data lines D.sub.1 to D.sub.n arranged in a direction orthogonal to the gate lines each spaced apart from one another by another fixed interval, and a plurality of unit pixel regions each formed within a space region at the intersections of the gate lines N.sub.1 to N.sub.n and the data lines D.sub.1 to D.sub.n.
Each of the unit pixel regions includes a pixel electrode (not shown) and a thin film transistor 10 having a gate electrode 18 connected to a gate line, a drain electrode connected to the data line, and a source electrode 22 connected to the pixel electrode, for applying data signals to the pixel electrode.
The unit pixel region also includes a stacked type capacitor C.sub.ST and a liquid crystal capacitor C.sub.LC, as seen in FIG. 3. C.sub.ST exists between the pixel electrode and the adjacent gate line and compensates for the leakage of applied data signal charges. C.sub.LC results from the pixel electrode on the underside plate and a common electrode 24 (FIG. 1) on the upper plate.
The common electrodes 24 are integrated into a single electrode on the upperside plate and, though not shown, the gate lines N.sub.1 to N.sub.n and the data lines D.sub.1 to D.sub.n are connected to a gate operating IC and data operating IC, respectively, and are operated by each of the respective IC's.
Operation of the conventional liquid crystal display having the foregoing system will be explained as follows.
Upon sequential application of gate operation pulses to each of the gate lines N1 to N.sub.n by the gate operation IC, each of the thin film transistors 10 having the gate operation pulses (voltage) applied thereto is turned on. During the time the thin film transistor 10 is turned on, the pixel 5 displays the image signals based on data voltages, containing image information, that are applied from the data operation IC to the liquid crystal through the thin film transistors 10 via the data lines D.sub.1 to D.sub.n.
At this time, common voltage V.sub.COM is applied to the common electrode 24 where the common voltage V.sub.COM is alternating or direct voltage which is a central value for a pixel electrode swing. As used in this disclosure, pixel electrode swing is the periodic alternations of the pixel electrode voltage between a high and a low voltage as compared to the voltage of the common electrode. Pixel electrode swing is used to prevent damage to the liquid crystal that would occur if a constant high or low voltage were applied.
When the common voltage and gate operation voltages are applied, the thin film transistor, turned on when the gate operation pulse is rising, charges the capacitors C.sub.ST and C.sub.LC with the data voltage. This charged voltage is maintained until the next field, even if the thin film transistor is turned off at the termination of the gate operation pulse.
However, the conventional liquid crystal display experiences problems as described below.
Since the described thin film transistor 10 has the gate electrode 18 overlapping the source electrodes 22 and drain electrodes 20, with the parasitic capacitors C.sub.gs and C.sub.gd formed between them, the pixel voltage drops .DELTA.V.sub.p (FIG. 2) because of the capacitive coupling when the thin film transistor is turned off the termination of the gate operation pulse.
That is, as shown in FIG. 3, when the operating pulse on n.sup.th gate line is dropping (e.g., when the gate operating pulse is changing from 15 V to -10 V) at 0 V of common voltage, and 5 V data voltage is charged in the pixel electrode, voltage to both ends of a parasitic capacitance C.sub.gs (formed by the gate electrode and the source electrode) has a change of 25 V. This change of voltage at both ends of the C.sub.gs, compensated with the capacitors C.sub.ST and C.sub.LC, drops the voltage of the pixel electrode by a value .DELTA.V.sub.p.
This drop in voltage results in crosstalk on the visual display. Crosstalk may occur when a window pattern is to be displayed by charging a line from which the window pattern starts. Since a parasitic capacitance is formed between the data line and the common electrode and the pixel electrode voltage drops by .DELTA.V.sub.p, the pixel cannot have exact data voltage charged therein.
This voltage drop at the pixel electrode degrades the picture quality because the voltage drop causes flicker of images and affects the common voltage terminal on displaying the window pattern to cause a phenomenon in which a grey scale of the image is changed (horizontal cross talk) until the voltage on the common voltage terminal is stabilized.